The present invention describes a system and method for locating and tracing multiple sound sources that can be stationary or moving in space and visualizing the resultant sound pressure fields in 3D space in real time.
Presently, there are no known systems and tools that enable one to visualize the sound pressure field produced by arbitrary (stationary/moving) sound sources in 3D space in real time. There are systems and tools available, however, to identify a sound source using the beamforming technology, and systems and tools to visualize a 3D sound field via nearfield acoustical holography (NAH) technology separately.
Typically, systems and tools based on beamforming technology require the use of a camera and an array of 30-60 microphones to measure the sound pressure, and then overlay the high sound pressure spots on the image of a test object captured by the camera to indicate the locations from which sounds are emitted.
The underlying principle behind beamforming is a delay and sum technique. By changing the time delays, namely, the phases of sound signals in the individual microphone channels and bringing all of them to be in phase so as to constructively reinforce each other, one can form a peak sound pressure, i.e., a beam that points in the direction of sound wave propagation in the space. This delay and sum process is equivalent to rotating the microphone array until it is in line with the incident sound wave. By using a camera and taking a picture of a test object that creates sound, one can overlay the high sound pressure on the image of the test object to indicate where sound is emitted. Note that since beamforming relies on a plane wave assumption, it can only reveal the direction of wave propagation but not the depth information, i.e., the distance of a sound source. The use of a camera compensates this shortcoming as a camera image is 2D, so the depth information is automatically suppressed.
In reality most source sources are 3D with complex geometry. Therefore, the acoustic information offered by beamforming is usually quite limited. Moreover, the sound pressure graph provided by beamforming is on a 2D measurement surface, but not on a 3D source surface. In particular, beamforming is effective for impulsive and broadband sound signals that contain high frequency components. In fact, the higher the frequency content and the broader the frequency bands are, the higher the spatial resolution of beamforming is. This is because the spatial resolution of beamforming is no better than one wavelength of a sound wave of interest, so it cannot discern two sources separated by a distance less than one wavelength. Hence beamforming is not suitable for low frequency cases. Also, the delay and sum technique is not applicable for locating sinusoidal, narrowband or tonal sound source. Finally, beamforming can not be used to monitor multiple sound sources in motion simultaneously.
NAH enables one to obtain 3D images of a sound field and very detailed and accurate information of the acoustic characteristics of a complex structure, including the source locations. However, NAH requires taking measurements of the acoustic pressures via an array of microphones positioned at a very close distance around the entire source. In particular, if a 3D image of a sound field is desired, measurements should include not only the source surface, but also the reflecting surfaces including floor, surrounding walls and ceiling, which is unfeasible in engineering applications. Finally, the state-of-the-art NAH does not allow for visualization of a 3D image of a sound field in real time. All visualization must be done in post processing.